Objective: To quantify the effect of selection of relatively healthy women in studies reporting reduced relative risk for cardiovascular disease in postmenopausal women taking hormone replacement therapy.
Design: Review of the follow up studies reported in three recent meta-analyses to determine the effect of oestrogen therapy on both total cancer and cardiovascular disease. The same standard statistical methods as in the original analyses were used.
Main outcome measures: Relative risks of total cancer and cardiovascular disease.
Results: In most of the follow up studies the relative risk for total cancer was below 1. The studies that showed the largest reduction in cardiovascular disease also showed the largest reduction in cancer, indicating a healthy cohort effect. Although heterogeneity within the studies prevented pooling, the best estimate for the protective effect on total cancer was a relative risk of 0.83 among women taking oestrogen (95% confidence interval 0.71 to 0.96), while in the same studies the relative risk for cardiovascular disease was 0.57 (0.50 to 0.64).
Conclusions: Unintended selection of relatively healthy women for oestrogen therapy may have influenced the reported beneficial effect of oestrogen therapy on cardiovascular disease. It is unclear how much of the cardioprotection is due to this selection. Universal preventive hormonal replacement therapy for postmenopausal women is unwarranted at present.